Photoaffinity labeling of an herbicide receptor protein in chloroplast membranes

2-Azido-4-ethylamino-6-isopropylamino-s -triazine (azido-atrazine) inhibits photosynthetic electron transport at a site identical to that affected by atrazine (2-chloro-4-ethylamino-6-isopropylamino-s -triazine). The latter is a well-characterized inhibitor of photosystem II reactions. Azido-atrazine was used as a photoaffinity label to identify the herbicide receptor protein; UV irradiation of chloroplast thylakoids in the presence of azido[14 C]atrazine resulted in the covalent attachment of radioactive inhibitor to thylakoid membranes isolated from pea seedlings and from a triazine-susceptible biotype of the weedAmaranthus hybridus . No covalent binding of azido-atrazine was observed for thylakoid membranes isolated from a naturally occurring triazine-resistant biotype ofA. hybridus . Analysis of thylakoid polypeptides from both the susceptible and resistantA. hybridus biotypes by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, followed by fluorography to locate14 C label, demonstrated specific association of the azido[14 C]atrazine with polypeptides of the 34- to 32-kilodalton size class in susceptible but not in resistant membranes.